1. Field of the Invention
The present invention relates generally to apparatus for controlling the operation of extrusion apparatus, and more particularly to an improved circuit for controlling the movement or position of a pintle relative to an extrusion die head.
2. Description of the Prior Art
In the manufacture of a blow-molded article, such as a bottle or the like, a parison is initially extruded through a variable orifice between a movable pintle and a die head, and is subsequently inflated to the desired configuration in a blowmold. When designing the shape of the initial parison, one should consider the shape of the ultimate blow-molded article. Hence, the wall thickness of the parison may vary considerably along its extruded length. This is accomplished in known extrusion apparatus by providing a pintle or mandrel which is mounted for movement relative to a die head so as to vary the width of the annular gap or orifice therebetween. Hence, the pintle may be selectively moved relative to the die head according to a programmed sequence, to provide a parison profile having a desired wall thickness at different locations along its extruded length.
German Offenlegungsschrift No. 2,445,394 (corresponding to U.S. Pat. No. 3,865,528) discloses a method of producing an electrical control signal for controlling the position of the pintle relative to the die head. This known method provides an electrical control circuit which includes an impedance matrix with a plurality of potentiometers, which may be used to produce a progression approximating the desired control function. This recognizes that a progression can be formed by superimposing triangular signals which overlap one another in time. For this purpose, triangular voltages are successively supplied to the potentiometers of the impedance matrix. The height of each triangle varies with the voltage appearing at the middle tap of the potentiometer. The modified voltages of adjacent triangles are then summed by superposition. In this known structure, each potentiometer is provided with its own individual signal generator, which produces a wave form in the form of a plurality of continuous voltage triangles. Each signal generator is supplied with the command signal, which is either derived directly from the pintle position, or independently generated and synchronized with the cycle of the pintle movement.
A drawback of this known control circuit is that a separate signal generator is required for each potentiometer in the impedance matrix. For example, for a twenty-five point programmer (i.e., one capable of producing a polygon with twenty-five corners), twenty-five of such signal generators are required. Since a single signal generator is relatively expensive, the entire control circuit becomes both expensive and unnecessarily large. In addition, tolerances between individual signal generators have a delitescent effect on total circuit function and performance.